The present invention relates to a pulse width modulation method for controlling a power converter, as well as to a pulse width modulator, a power conversion method, and a power converter.
In a power converter which converts a DC current into an alternating current by pulse width modulation, a method of detecting the electric current of the AC side from the electric current of the DC side is described in the publication “Derivation of motor line-current waveforms from the DC-link current of an inverter” IEEE PROCEEDING, Vol.136, Pt. B, JULY 1989, pp. 196-204.
Moreover, a method disclosed in the Japanese Patent Application Laid-Open No. 2001-327173 provides pulse width modulation suitable for the method of detecting this electric current.
In the technique described in the literature “Derivation of motor line-current waveforms from the DC-link current of an inverter” IEEE PROCEEDING Vol. 136, Pt. B, July 1989, pp. 196-204, it is difficult to detect stably the electric current of the DC side because the electric current pulse width of the DC side becomes small when the AC voltage is at a low level.
In the Japanese Patent Application Laid-Open No. 2001-327173, the electric current pulse width of the DC side is corrected so as to have enough width for steady detection of the electric current pulse width in the first half of one cycle of carrier wave for the pulse width modulation. Moreover, the AC side voltage is corrected to become the desired voltage in the latter half. Therefore, a pulse width necessary for detecting the electric current is not secured in the latter half. Moreover, it is necessary to operate the modulated wave at a cycle of one half of the carrier wave cycle. Therefore, a highly efficient microcomputer is necessary for the operation. Further, an electric current pulse width necessary to detect the electric current might not be able to be secured even in the Japanese Patent Application Laid-Open No. 2001-327173.
In the method according to the Japanese Patent Application Laid-Open No. 2001-327173, a normal switching cannot be occasionally done though the switching of a certain phase and the switching of another phase will be done in a short time due to the mutual interference generated at this time.
As mentioned above, in the case that the output current of the power converter is detected from DC current Idc in the conventional power converter which uses the pulse width modulation, for converting the DC current into the AC current, it is difficult to detect because the pulse width of DC current Idc becomes narrow when the voltage instruction signal between phases is small.